The present invention relates generally to a control system for a multiple compressor refrigeration or air conditioning system. Specifically, the present invention relates to a control system that determines when to start additional compressors in a multiple compressor refrigeration or air conditioning system during an initial pull down operation of the refrigeration or air conditioning system.
In a refrigeration system that uses a chilled liquid, the chilled liquid is circulated through a building or area to remove heat from the building and cool the building. When cooling is no longer required in the building, the refrigeration system is shut down and the previously chilled liquid that cooled the building is permitted to warm to ambient or close to ambient temperatures. When cooling is again required in the building, the temperature of the liquid to be circulated through the building has to be pulled down from an elevated temperature to the appropriate operating setpoint temperature for effective cooling of the building. This process of chilling the liquid that is circulated in a building from an elevated temperature to the operating setpoint temperature is commonly referred to as a pull down operation.
In a multiple compressor refrigeration or chiller system, it is common to cycle the compressors in order to match the chiller system capacity to the building cooling load. Some techniques used to evaluate and control chiller system capacity can include comparing the leaving chilled liquid temperature, i.e., the temperature of the liquid from the evaporator used to cool the building, to a desired operating setpoint temperature and/or comparing the compressor motor power to the maximum compressor motor power. Both of these techniques can be effective to provide adequate control of the chiller system when the chiller system is operating in a steady state mode. However, these techniques may provide a false indication of the need for additional chiller system capacity during a pull down operation. For example, during a pull down operation the difference between the leaving chilled liquid temperature and the operating setpoint temperature is often large, which large difference in temperatures would indicate the need for additional system capacity even though the currently operating compressor(s) may provide more than enough system capacity for the building cooling load. This false indication can occur when the currently operating compressors have not yet had time to pull down the leaving chilled liquid temperature to the operating setpoint temperature.
Some potential problems with having too much chiller system capacity during a pull down operation include the possibility of overshooting the operating setpoint temperature and the possibility of frequent cycling on and off of the compressor motors. An overshoot of the operating setpoint temperature occurs when the leaving chilled liquid temperature continues to decrease past the operating setpoint temperature. If the leaving chilled liquid temperature becomes too low, the liquid in the evaporator may start to freeze which can reduce system efficiency and potentially cause damage to the chiller system. The frequent cycling on and off of compressor motors is also undesirable because it results in greater energy consumption by the chiller system. Furthermore, in very large chiller systems using very large chiller motors, there may be limits placed on the starting of the compressor motors, which limitations can result in a compressor not being started even though there is a demand for additional chiller system capacity. One example of where a motor may not be able to be started can occur when an additional compressor is cycled on for the pull down operation, is cycled off once the operating setpoint temperature has been reached, and then is needed to be cycled on again for steady state operation of the chiller system to satisfy the building cooling load but cannot be cycled on because of a limitation on the number of starts of the compressor motor.
Therefore, what is needed is a control algorithm that can determine when a current compressor configuration in a multiple compressor refrigeration or chiller system is inadequate to pull down the leaving chiller liquid temperature to the desired operating setpoint temperature and can start an additional compressor in the multiple compressor refrigeration system to assist in the pull down of the leaving chiller liquid temperature to the desired operating setpoint temperature without unnecessary cycling of the additional compressor.
One embodiment of the present invention is directed to a method for determining when to start additional compressors in a multiple compressor chiller system during a pull down operation of a leaving chilled liquid temperature in the multiple compressor chiller system. The method includes the step of measuring a parameter of a multiple compressor chiller system. The measured parameter is related to a leaving chilled liquid temperature of the multiple compressor chiller system. The method also includes the steps of calculating a rate of change of the measured parameter of the multiple compressor chiller system and comparing the calculated measured parameter rate of change with a predetermined rate of change for the measured parameter. Finally, the method includes the step of starting an additional compressor in the multiple compressor chiller system in response to the calculated measured parameter rate of change being less than the predetermined rate of change for the measured parameter.
Another embodiment of the present invention is directed to a method for controlling a pull down operation of a secondary liquid leaving an evaporator in a multiple compressor refrigeration system from an elevated temperature to a setpoint temperature. The method includes operating a predetermined number of compressors in a multiple compressor refrigeration system in response to a temperature of a secondary liquid leaving an evaporator in the multiple compressor system being elevated. The operation of the predetermined number of compressors pulls down the temperature of the secondary liquid leaving the evaporator toward a setpoint temperature. Next, a parameter of the multiple compressor system related to the to the temperature of the secondary liquid leaving the evaporator is measured and a rate of change of the measured parameter is determined. The determined measured parameter rate of change is compared with a predetermined rate of change for the measured parameter and an additional compressor in the multiple compressor refrigeration system is operated in response to the determined measured parameter rate of change being less than the predetermined rate of change for the measured parameter. The operation of the additional compressor assists the predetermined number of compressors in pulling down the temperature of the secondary liquid leaving the evaporator toward the setpoint temperature.
Still a further embodiment of the present invention is directed to a computer program product embodied on a computer readable medium and executable by a microprocessor for determining when to start additional compressors in a multiple compressor chiller system during a pull down operation of a leaving chilled liquid temperature in the multiple compressor chiller system. The computer program product includes computer instructions for executing the step of measuring a parameter of a multiple compressor chiller system. The measured parameter is related to a leaving chilled liquid temperature of the multiple compressor chiller system. The computer program product also includes steps for executing the steps of determining a rate of change of the measured parameter of the multiple compressor chiller system and comparing the determined measured parameter rate of change with a predetermined rate of change for the measured parameter. Finally, the computer program product includes computer instructions for starting an additional compressor in the multiple compressor chiller system in response to the determined measured parameter rate of change being less than the predetermined rate of change for the measured parameter.
One advantage of the present invention is that it extends the refrigeration or chiller system""s service life by limiting the number of starts of the compressor motors of the refrigeration system.
Another advantage of the present invention is that it can provide energy savings and avoid overshoot of a setpoint temperature by conducting the pull down operation at an appropriate rate.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.